The present invention relates generally to optical scanners and in particular to a device for reading photo-stimulable storage luminescent substances.
The present invention uses the effect of photo-stimulated luminescence. Phosphorus is one example of the substances which display photoluminescence. A storage luminescent substance, e.g. one which contains phosphorus, absorbs radiation energy, which excites electrons into higher energy states. These higher energy states are unstable, so that the phosphor electrons fall back into a state with lower energy, the energy difference being emitted as light. The emitted light energy typically has a different wavelength than the radiation energy which has induced the photoluminescence.
In the field of dental medicine in particular, in which a plurality of X-ray pictures is created, there is a great need to replace the traditional X-ray pictures with new imaging techniques. It is time consuming and expensive to first expose and then develop X-ray films, the time needed for exposure and development being in general considerably greater than the time spent in actually looking at the X-ray picture in order to make a diagnostic decision. For this reason X-ray storage luminescent substance foils have recently been used instead of the traditional X-ray films, which have to be exposed and then developed, e.g. in the field of dental medicine. As has already been mentioned, such storage luminescent substance foils comprise an storage photoluminescent substance, e.g. phosphorus, which is capable of retaining a pattern over several days after it has been exposed. Multiple exposure and a higher dynamic range are also advantageous attributes. Image data obtained e.g. by exposure to radioactive substances can be recalled using light of the appropriate wavelength. In particular, light of the appropriate wavelength stimulates or excites photoluminescence in the storage luminescent substance, the light emitted by the excited storage luminescent substance typically having a different wavelength than that of the exciting light.
A glass fibre scanner for scanning an storage phosphor imaging plate is known from EP 0559118 A1. The glass fibre scanner comprises a forklike glass fibre bundle with emitter fibres and collecting fibres. In addition a device for focusing light into the near end of the emitter fibres is provided, while furthermore a device for focusing light from the far end of the emitter fibres onto the phosphor imaging plate and a device for collecting light at the far end of the collecting fibres are used to increase the sensitivity and wavelength resolution of the arrangement. Such a glass fibre cable is used in the x-y scanner to scan an storage phosphor imaging plate so as to transport light from a light source to the phosphors in the storage phosphor imaging plate and to collect phosphorescence induced by the light.
The glass fibre scanner for scanning an storage phosphor imaging plate according to EP 0559118 A1 requires a focusing/imaging lens, which is connected to the far ends of the at least one emitter fibre and the at least one collecting fibre. This focusing/imaging lens, which has to serve both to focus the exciting light and to collect the light created by photoluminescence, makes the whole arrangement more expensive, since it must not only be fabricated but it must also be positioned and adjusted.
JP-1-185503 discloses a photodetector with a light receiving surface which is contacted directly by a receiving glass fibre with a core and a sheath. The materials of the core and the sheath of the glass fibre are so chosen that the refractive indices of the same decrease at different rates as the wavelength increases, whereby the numerical aperture of the receiving glass fibre is wavelength dependent and decreases in the direction of increasing wavelength. As a result the receiving glass fibre provides relatively good transmission conditions for the light emitted by a phosphor layer whereas it provides relatively poor transmission conditions for the light for exciting the phosphor, this light having a longer wavelength than the phosphorescent light emitted by the phosphor layer, whereby the glass fibre acts roughly like a high-pass filter. Direct connection of the glass fibre with the light receiving surface of the photodetector is thus possible.
DE 2363995 C2 discloses a method for creating a radiographic picture and a device for performing this method. Through the agency of a radiation source light is here projected through an interference filter onto a region of an excitable medium, whereupon the excitable medium emits light radiation which is reflected at the interference filter and is projected through a lens onto an input area of an image intensifier tube.
U.S. Pat. No. 5,557,452 discloses a confocal microscope. Light radiated from one end of a glass fibre is here focused by a microscope objective and radiated through a window onto a sample to be investigated, the result being that the sample fluoresces. Part of the fluorescent light radiated by the sample passes through the window again and is fed into a glass fibre by the microscope objective to be processed further.
It is the object of the present invention to provide an arrangement for reading photo-stimulable storage luminescent substances and a glass fibre drum scanner which can be manufactured less expensively.
In accordance with a first object of the invention, this object is achieved by an arrangement for reading photo-stimulable storage luminscent substances comprising: an excitation glass fibre into which light which excites the storage luminscent substance can be fed by means of a light source; and a receiving glass fibre into which the light produced by an excited storage luminescent substance can be fed, wherin that end of the excitation glass fibre which is positionable so as to be close to the storage luminescent substance is arranged next to that end of the receiving glass fibre which is positionable so as to be close to the storage luminscent substance, wherein the excitation glass fibre has a first numerical aperture, and wherein the receiving glass fibre has a second numerical aperture which is large compared with the first numerical aperture, whereby the light which is fed out of the excitation glass fibre is directed straight onto the storage luminscent substance without an optical arrangement.
In accordance with a second aspect of the present invention, this object is achieved by a glass fibre drum scanner comprising: a glass fibre arrangement for reading photo-stimulable storage luminescent substances comprising an excitation glass fibre into which light which excites the storage luminescent substance can be fed by means of a light source; and a receiving glass fibre into which the light produced by an excited storage luminescent substance can be fed, wherein that end of the excitation glass fibre which is positionable so as to be close to the storage luminescent substance is arranged next to that end of the receiving glass fibre which is positionable so as to be close to the storage luminescent substance, wherein the excitation glass fibre has a first numerical aperture, and wherein the receiving glass fibre has a second numerical aperture which is large compared with the first numerical aperture, whereby the light which is fed out of the excitation glass fibre is directed straight onto the storage luminescent substance without an optical arrangement; and excitation light source whose light can be fed into another end of the excitation glass fibre; a receiver unit for receiving light transmitted in the receiving glass fibre; a drum unit rotatable about a first axis and to the curved surface of which the storage luminescent substance can be affixed; an advancing device for moving the closely arranged ends along the first axis; and a control unit for synchronizing operation of the excitation light source, the receiver unit, the drum unit and the advancing device.
The present invention is based on the finding that it is possible to achieve a more robust and more economical glass fibre drum scanner by dispensing with the optics for focusing the excitation light and for receiving the excited light so as to achieve a readout with high resolution and high contrast. The optics for focusing and for receiving the excited light can the more readily be dispensed with if a glass fibre with a small numerical aperture is used for the excitation glass fibre, whereas, by contrast, the receiving glass fibre must be a glass fibre with a large numerical aperture. The numerical aperture is known to specialists and is equal to the sine of half the solid angle of a maximum radiation cone which can be supplied by a glass fibre or which can be fed into a glass fibre. For a graded-index fibre e.g. the numerical aperture is calculated from the square root of the difference between the mean refractive indices of the glass fibre sheath and the glass fibre core. In accordance with the present invention the excitation fibre therefore consists of a glass fibre with a small numerical aperture, whereby the excitation light cone which it produces has a small solid angle, which means that, for a still moderate separation of the end of the excitation glass fibre which is positioned near the storage luminescent substance, a sufficient spot size is attainable, which in the final analysis defines the resolution with which the previously exposed storage luminescent substance foil can be read. By contrast, the receiving glass fibre, i.e. the glass fibre which collects the light emitted due to photoluminescence, must have a large numerical aperture, greater at least than the numerical aperture of the excitation glass fibre, in order that as much as possible of the light released due to photoluminescence can be collected. To increase the amount of photoluminescent light which is detected, and thus increase the sensitivity of a scanner, a multiplicity of receiving glass fibres may for preference be arranged around the excitation glass fibre, it being obvious to persons skilled in the art that the numerical aperture of a receiving glass fibre of the multiplicity of receiving glass fibres may then be smaller than the numerical aperture of a single receiving glass fibre while still achieving sufficient sensitivity of the storage luminescent substance scanner.